The invention relates to a method for the production of phosphoric acid with high contents of fertilizer-nutrients and of filterable calcium sulfate-semihydrate through decomposition of phosphate rock with nitric acid with the addition of sulfate in the form of ammonium sulfate or sulphuric acid while adhering to the optimal requirements for the precipitation of calcium sulfate-semihydrate in regard to acid-concentration and working temperature. The filtrate of the decomposition can--by adding further nutrients or decomposition material--be worked into multinutrient-fertilizers in the usual manner.
When producing multi-nutrient-fertilizers with the hitherto customary nitro-phosphate method, rock phosphate is decomposed with a mixture of nitric- and phosphoric acid, i.e. with a nitrogen-containing phosphoric acid, whereby, subsequently, this decomposed mixture is neutralized by adding ammonia. Afterwards, in a given case, after the addition of potash- or other salts, the mixture is granulated. The nitro-phosphate method is easy to handle but it has the disadvantage that phosphoric acid must be used, the reltatively high price of which has a negative effect on the economics of the end product.
In order to decrease these economic disadvantages, various methods for the production of multi-nutrient fertilizers have become known which work with decreased use or no use of phosphoric acid.
It is known to carry out the rock phosphate decomposition only with nitric acid and to subsequently separate through cooling a part of the calcium ions in the form of calcium nitrate, which then can be used for the production of calcium ammonium nitrate (Odda-process).
It is also known to precipitate the calcium ions which result from the nitric acid decomposition of rock phosphate with ammonium sulfate in the form of gypsum, CaSO.sub.4 .times.2H.sub.2 O, whereby the gypsum is usually rejected as waste product and the developing nitrogen-containing phosphoric acid is worked further into multi-nutrient fertilizers. This method has the advantage that ammonium sulfate can be used whose sulfate part is usually cheaper than that of the technical sulphuric acid, so that one can save on expenditures in this manner.
This known method makes a complicated purification of the gypsum from the soluble sulfates necessary. For this considerable amounts of water are necessary which later on must be evaporated again during the production of fertilizers. The filtering area required with this method is extremely high, 2.5-5 m.sup.2 /t gypsum. On the other hand, nutrient losses occur with insufficient washing out. As already mentioned, the remaining clinging impurities usually influence the thus obtained gypsum to such an extent that it no longer can be used for building purposes.